Workpiece drive means for turning lathes

ABSTRACT

The lathe includes opposed bodily shiftable headstocks each having a rotational spindle for engaging opposite ends of a workpiece as the headstocks are shifted toward one another. The spindles are rotated by means of two identical hydraulic pumpmotors connected in a pressurized fluid piping system wherein fluid is kept in circulation by a third identical hydraulic pumpmotor driven by a constant speed electric motor. By including the spindle-driving pump-motors in a common fluid system, said pumpmotors are obliged to share equally the work of driving opposite ends of an elongate workpiece supported between centers on the spindles. The drive means between each spindle and one end face of the work piece, is a purely frictional drive means the effectiveness of which is aided by a high-pressure auxiliary means to elevate and maintain the force of frictional contact between the workpiece end and a unique friction-ring assembly on the spindle involving hard and soft polyurethane transmission members. The pump-motors, two of which operate as spindle drivers, while one operates as a pump delivering operating fluid to the others, are all identical and interchangeable with one another in the interests of economical maintenance and speedy replacements.

United States Patent Dugle [54] WORKPIECE DRIVE MEANS FOR TURNING LATHES[72] Inventor: Thomas E. Dugle, Cincinnati, Ohio [73] Assignee: PlanetProducts Corporation, Cincinnati,

Ohio [22] Filed: May 27,1970

[2]] Appl.No.: $1,413

Related US. Application Data [62] Division of Ser. No. 7l0,l87, Mar.4,1968, Pat. No.

[52] U.S.Cl ..82/8 [51] Int. Cl. ...B23b 5/28 [58] Field olSearch ..82/8

[56] References Cited UNITED STATES PATENTS 3,079,822 3/1963 Miller etal ..82/8 X 2,823,493 2/1958 Stanley 1,522,674 l/l925 Fickett ..82/8

FOREIGN PATENTS OR APPLICATIONS 874,978 4/1953 Germany ..82/8

1,104,675 6/1955 France ..82/8

Primary Examiner-Leonidas Vlachos Attorney-J. Warren Kinney, Jr.

[ ABSTRACT The lathe includes opposed bodily Sllllftfiblfi headstockseach having a rotational spindle for engaging opposite ends of aworkpiece as the headstocks are shifted toward one another. The spindlesare rotated by means of two identical hydraulic pump-motors connected ina pressurized fluid piping system wherein fluid is kept in circulationby a third identical hydraulic pump-motor driven by a constant speedelectric motor. By including the spindle-driving pump-motors in a commonfluid system, said pumpmotors are obliged to share equally the work ofdriving opposite ends of an elongate workpiece supported between centerson the spindles. The drive means between each spindle and one end faceof the work piece, is a purely frictional drive means the effectivenessof which is aided by a high-pressure auxiliary means to elevate andmaintain the force of frictional contact between the workpiece end and aunique friction-ring assembly on the spindle involving hard and softpolyurethane transmission members. The pumpmotors, two of which operateas spindle drivers, while one operates as a pump delivering operatingfluid to the others, are all identical and interchangeable with oneanother in the interests of economical maintenance and speedyreplacements.

12 Claims, 4 Drawing Figures 2 5 7 -20 28 J L 2 74 l IL 48 90 T e 92 soi \Y\ lf.. 4 j

PATENTED PR 25 m2 3, s57, 95 0 SHEET 1 BF 2 FIG. I

INVENTOR THOMAS E. DUGLE WORKPIECE DRIVE MEANS FOR TURNING LATIIES Thisis a division of application Ser. No. 710,187, filed Mar. 4, 1968, nowU.S. Pat. No. 3,605,533.

Thepresent invention relates to improvements in a workpiece drive meansfor turning lathes, and is applicable particularly in the machining ofheavy workpieces such as railway car axles or the like which have ratherlarge and substantially flat end faces with axial sockets receptive ofthe usual lathe spindle centers. The invention is useful in themachining of new or used axles or other appropriate workpieces whichlend themselves to handling by the procedure hereinafter to bedisclosed.

The disclosure will be directed to the machining of a railway car axleas a mere example of a workpiece type that may be accommodated, and withthe understanding that the invention is not to be considered limited tocar axle machining.

An object of the invention is to facilitate and expedite the mounting ofa heavy workpiece between centers of a lathe, and completing the driveconnection for turning the workpiece.

Another object of the invention is to provide simple, durable andeffective means for transmitting rotary motion of the lathe spindle tothe workpiece, without objectionable slippage and without the use ofclamps which may mark the finish of the workpiece.

A further object ofthe invention is to provide a highly effectivefriction drive means for rotating workpieces in the course ofa machiningor finishing operation.

Another object is 'to provide improved highly effective means forfriction driving opposite ends of a workpiece supported between centersof two headstocks.

A still further object of the invention is to provide an improved meansfor hydraulically rotating the spindles of two headstocks of a lathe,with equal distribution of driving forces at opposite ends ofa workpiecesupported between the headstocks.

Another object is to provide improved means for comprising a workpiecebetween centers oftwo driving spindles.

The foregoing and other objects and advantages are attained by the meansdescribed herein and illustrated upon the accompanying drawings, inwhich:

P10. 1 is a vertical cross-section of one end of a turning lathe,disclosing the improvements ofthe present invention.

FIG. .2 is a diagrammatic view of a hydraulic drive system for twoopposed headstock spindles ofthe lathe shown in FIG.

FIG. 3 is an end elevation, taken on line 3-3 of FIG. 4, showing africtional drive means for a workpiece, applied to a headstock spindle.

FIG. 4 is a fragmentary vertical cross-section taken on the spindle'axisof FIG. 3.

1n the machining of heavy workpieces such as railway can axles with thewheels removed, it is desirable to support and rotationally drive bothends of the workpiece with equalized torque forces, and without the useof chucking devices which might mark or indent the finish of theworkpiece. It is therefore desirable-to transmit rotation of thespindles to opposite ends of the workpiece by frictional means abuttingthe workpiece ends with great force, and to do this with safety andfacility, notwithstanding the possible presence of irregularities in theworkpiece end faces. Inasmuch as machining of the journal bearings andwheel seats of an axle imposes considerable resistance to rotation ofthe axle, any frictional driving means therefor must be durable andhighly effective in resistingslippage and lateral displacement of theaxle.

With reference to FIG. 1,.the lathe comprises a bed 8 having ways tosupport a headstock 10 which is slidable lengthwise a limited distancealong the bed. Behind the headstock and upon the bed is fixed a subheadcomprising housing 12, which contains power means for axially shiftingthe headstock and spindle 14, as is necessary in the placement andremoval of workpiece Welt must be understood that the lathe bed carriestwo headstocks, one near each end of the bed, and each of whichheadstocks includes a driving spindle operative to rotate the workpiece.The workpieces have, or may be caused to have coaxial sockets at theopposite ends thereof to accom modate the spindle centers 16 of bothheadstocks. The center 16 may enter a socket more deeply then thedrawing indicates. Both centers are to be spring-loaded, as at 17.

From the foregoing explanation, it will be understood that the workpieceis to be supported between centers oftwo headstocks while undergoingmachining, and upon completion of the machining operation the headstocksand their spindles may be retracted to release the workpiece which thenmay be lifted away from the work position and conveyed from the vicinityofthe machine. workpieces in the form of railway car axles may weightmore than 1,000 pounds.

Spindle 14 may be journalled for rotation in suitable bearings 18, 18supported by the headstock, and at one end of the spindle may be fixed agear 20 in mesh with the driving pinion 22 of a hydraulic pump-motor 24carried by the headstock. The forward end of the spindle carries anadapter 26 (FIG. 4), which adapter is formed with a flange 28 and anannular shoulder 30. Adapter 26 rotates with the spindle l4, andprovides support for the spindle.

About shoulder 30 and in abutment against flange 28 is positioned aremovable drive ring 32. To assure rotation of the drive ring withdriving adapter 26, the adapter may be provided with one or more fixedstuds each having a head 34 projected into a socket 36 formed in a rearface 38 of the drive ring. To preclude inadvertent axial displacement ofthe drive ring from shoulder 30, a snap ring 40 or equivalent means maybe employed for detachably joining the: parts.

1n the forward face 42 of drive ring 32 is formed an annular groove 44in which is mounted the means previously mentioned for frictionallydriving the workpiece W. Said means comprises a completely confinedback-up ring or pad 46, and a superposed load ring or pad 48 whichextends slightly beyond face 42 to abut an end face of the workpiece Wunder high pressure. This abutting relationship between ring 48 and anend face of the workpiece, constitutes the friction drive means for theworkpiece.

The load ring 48 is to be formed of a hard and durable material which isrelatively incompressible, like a ring of steel or other metal. To alimited extent said ring may be flexible or bendable.

The load ring is to be backed up by a much softer ring 46, formed of amaterial having certain characteristics found in soft polyurethane, forexample. The back-up ring is required to act as a liquid or heavy greaseentrapped behind the load ring, in that slight tilting or bending of theload ring resulting from irregularities in the workpiece end face, willeffect a dis placement of the back-up material within the groove 44,resulting in substantially uniform distribution of pressure throughoutthe mass ofthe back-up material. The same effect might be obtained byfilling the space: behind the load ring with a multiplicity of ballbearings, which would act in much the same manner as a liquid, tocompensate for tilt or bending of the load ring pressed against aworkpiece end having a surface which may be wavy or slightly out ofsquare with the workpiece axis.

The soft back-up ring 46 may be formed of polyurethane, or a material ofthe class of soft polyurethane, having a hardness of the order of ShoreA-6O durometer. The material is substantially incompressible, thoughdeformable or fluid-like in performance under conditions of unequalforce application thereon. It is displaceable in all directions underlocalized application of pressure, and uniformly self-equalizing in muchthe manner of a liquid.

The hard ring or load ring 48, as before stated, is to be formed of ahard, durable, substantially incompressible material, having limitedflexibility and resilience characteristics by preference. The load ringhas an outer friction face to abut the workpiece, and under someconditions, the friction face might be roughened or otherwise treated toincrease the frictional grip of the load ring upon the workpiece.

A satisfactory and highly effective load ring may be formed of hardpolyurethane, or a material of the class of a polyurethane, having ahardness of the order of Shore D-70 durometer. Such material issubstantially incompressible, slightly flexible, very durable, and has ahigh coefficient of friction.

Ring 48 is precluded from rotation within groove 44, by a circular row47 of interfitting splines or keys formed in the groove 44 and upon anadjacent edge of ring 48.

The transmission rings 46 and 48 cooperate with one another to maintaina high-friction abutment against the end face of the workpiece,irrespective of irregularities that might be present in said end face.Sometimes the end face of the workpiece is found to be curved, orrippled, and in some cases the plane of the end face is not trulyperpendicular to the major axis of the workpiece. Such imperfections, inthe absence of correctional compensation as furnished by the differenttypes of rings 46 and 48, may cause a drive failure destrictive ofcutting tools and disruptive to production schedules. The cooperativehard and soft transmission rings supported by the drive ring asdisclosed, assure maintenance of a highly effective and reliablenon-slip frictional drive between the headstock spindles and oppositeends of the workpiece. Should the transmission rings ever requirereplacement, they may easily be removed from the groove 44; or ifpreferred, replacement may be effected by substituting a new drive ring32 for the one carrying worn transmission rings, this being easilyaccomplished by removing the snap ring 40.

Reverting to FIG. 1, it is clearly apparent that a bodily shifting ofheadstock 10 to the right will result in establishing the aforementionedfrictional drive for rotating the workpiece. Such shifting oftheheadstock may be performed by means of a screw 50 and a nut 52, arrangedto selectively vary the distance between the movable headstock 10 andthe fixed subhead 12.

Screw 50 has exterior threads as shown, and is in the form of a sleevehaving an elongate axial bore containing longitudinal internal splinesengaging the external splines 74 of a shaft 72. The sleeve screw may bemounted upon a stud end 56 of the spindle so as to have rotationalmovement independent of rotation of the spindle, or, immobility as thespindle rotates. The sleeve screw may impart longitudinal movement tothe spindle through a thrust bearing connection 58.

The internal threads of nut 52 are in constant engagement with theexternal threads of screw 50, and the nut may be integral with, orfixedly supported within, a non-rotative sleeve 60 to be shiftedlongitudinally by the action of hydraulic fluid under high pressure fedto a chamber 62 at the back of the sleeve. Fluid under high pressure maybe fed to chamber 62 through a tube 64 supplied by a pump 66 andcontrolled by a valve 68. Sleeve 60 acting as a piston may be keyed tohousing 12 as at 70, in such manner as to prevent rotation of the sleevewhile permitting limited axial shifting thereof. Axial shifting ofsleeve 60 as a piston bodily moves the headstock l and spindle 14.

The headstock and spindle are shiftable also by rotation of screw 50within the nut 52. Rotation of screw 50 may be effected by means ofapropeller shaft 72 having external splines 74 at one end thereof inengagement with the internal splines of the screw. Housing 12 supportsthe propeller shaft for rotation in suitable bearings 78. A sprocket 76is fixed to the rear end of shaft 72, and over the sprocket may betrained a chain or timing belt 80 driven by a second sprocket 82.

Drive sprocket 82 may be fixed upon a torque tube 84 which reachesbeyond the workpiece and terminates near the opposite end of the lathebed. The torque tube may be rotated at times by means of a reversiblefluid motor 86, through a transmission train 88 of any suitable type.Operating fluid may be supplied to motor 86 through a control valve 90and appropriate piping 92. As will be understood, the shaft of motor 86when rotated I in one direction will effect rotation of propeller shaft72 to advance screw 50, spindle l4, and headstock to the right in FIG.1; and conversely, an opposite direction of rotation of said motor shaftwill reversely drive the screw 50, to move the spindle and the headstockto the left.

As was previously mentioned herein, that end of the machine notillustrated upon FIG. 1, will be equipped with a slidable headstockidentical to headstock 10, and with second sub-head 12 which carries allthe elements necessary to shift the headstock by rotation of a propellershaft such as 72 (see FIG. 2). The second sub-head preferably issimplified to the extent of eliminating the fluid chamber 62 behind nutsleeve 60, so that said sleeve simply is fixedly secured to the secondsub-head.

The second sub-head may have its propeller shaft 72 driven from torquetube 84 in the same manner disclosed by FIG. 1, the two propeller shaftsthereby operating in synchronism to move the headstocks either towardone another to support the workpiece, or from one another to release theworkpiece. This presupposes, of course, screw threads ofopposite handsin the screw assemblies associated with the propeller shafts.

From the foregoing, it will be apparent that motor 86 may be energizedto effect movement of the two headstocks from one another to permitplacement of a workpiece W between the headstock centers. Then byreversing the direction of motor 86, the propeller shafts of theheadstocks may be rotated to move the headstocks toward one another,thereby to project the headstock centers into the end sockets of theworkpiece. The centers, being spring loaded, may yield to en sure thatthe end faces of the workpiece firmly abut the load rings 48 of thedrive rings 32, under the force of advancement ofthe headstocks by motor86.

Motor 86 will advance the headstocks to grip the workpiece primarilywith a force limited by the torque capacity ofmotor 86, and said motoraccordingly may stall in the effort to compress the workpiece withsufficient force to ensure an effective drive at the friction rings 48of the opposing headstocks. However, after the workpiece has beencompressed to the extent of the capability of motor 86, valve 68 may beopened to direct a secondary fluid under high pressure to chamber 62,for projecting the sleeve 60 with great force as required for theworkpiece machining operation. lf necessary, the secondary pressure inchamber 62 may be sustained throughout the machining operation. Also ifnecessary, the motor 86 may be kept under sustained driving pressure,although in practice this is found unnecessary if the threads at 50, 52are ofproper pitch.

Upon completion of the machining operation, the workpiece may bereleased by closing the valve 68 and exhausting the chamber 62, and byreversing the direction of motor 86 to retract screw 50 and theheadstock and spindle connnected thereto, at opposite ends ofthe lathe.

The hydraulic pump-motor 24 is driven by fluid under pressure circulatedthrough pipes or conduits 94 and 96 by an electric motor drivenhydraulic pump-motor 98, which latter is identical to pump-motor 24 butis driven by electric motor 100. The pipes or conduits have extensions102 and 104 which connect with a third hydraulic pump-motor 24' carriedby the headstock opposite to headstock l0. Said third hydraulicpump-motor is identical to those indicated at 24 and 98, and it drivesthe opposite end of the workpiece as previously explained.

Since the two headstock pump-motors 24 and 24' are identical in allrespects, and are connected in a common fluidpressure system, saidpump-motors will share equally the work of driving opposite ends of theworkpiece. That is, the system may be regarded as self-compensating asto the drive for opposite ends of the workpiece, which of course ishighly desirable.

The pump-motors 24, 24 and 98, are of the positive displacement type,and are variable as to speed in the system disclosed. All of saidpump-motors are sufficiently identical and interchangeable one withanother as pumps or as motors, so that in contemplation of a breakdownof any one of them, only one replacement need be kept on hand. This is amatter of economic importance, since the pump-motors are expensiveinventory items.

Although the drawings show a geared drive between the electric motor 100and the pump-motors 98 and 66, a direct drive here will often bepreferred. Also, these pump-motors may be driven by separate electricmotors if desired.

Since the pump-motors 98 and 66 and the driving means therefor are to bemounted elsewhere than upon the movable headstocks, as may well bepreferred, some portions of the fluid conduits shown will of course beof the flexible type, as will be understood. The fluid system employedin driving the spindles may be kept supercharged by incorporatingsuitable pump means, not shown.

It is to be understood that various modifications and changes may bemade in the structural details of the apparatus, within the scope of theappended claims, without departing from the spirit ofthe invention.

What is claimed is:

l. A lathe for machining a heavy elongate workpiece having oppositesubstantially planar end faces each provided with a socket coaxial withthe workpiece major axis, said lathe comprising: a stationary elongatebed having opposite end portions; a pair of headstocks slidablysupported near opposite end portions of the bed for shifting movementtoward and from one another; a drive spindle journalled for rotation ineach headstock, said spindles each having a forward spring loaded centerto enter a socket of a workpiece end face for support of the workpieceaxially between the spindles; means precluding axial shifting of thespindles relative to their respective headstocks; frictional drive meanson the spindles adjacent to the centers thereof, for frictionallyabutting the opposite end faces of the workpiece; motor means forrotating the spindles and the abutting workpiece; a pair of sub-headsfixed upon the bed outboard of the headstocks; a pair of interconnectedscrew-threaded members located between each sub-head and its adjacentheadstock, one of said members being mounted axially in abutment againstthe rear end of a spindle for rotation independently of the spindlerotation; the other of said screw-threaded members being non-rotatablymounted upon a sub-head in alignment with said one member; a rotatableshaft coaxial with said screw-threaded members and means on said shaftfor transmitting rotation of said shaft to said one member relative tosaid other member; power means reversible for rotating the shafts ofsaid sub-heads in unison, thereby to rotate said one screw-threadedmember of each spindle relative to said other interrelatedscrew-threaded member, for advancing and retracting the shiftableheadstocks relative to their respective adjacent sub-heads; andhigh-pressure hydraulic means selectively operative to augment the forceapplied to advance one of the headstocks by relative rotation of thescrew-threaded members associated with said one headstock.

2. The lathe as specified by claim 1, wherein the said highpressurehydraulic means is operative upon one of said screwthreaded members, forapplying auxiliary force axially to a spindle and a workpiece end faceabutting said spindle.

3. The lathe as specified by claim 1, wherein the said highpressurehydraulic means is operative upon the nomrotatable screw-threaded memberto shift the latter member in the direction ofa workpiece end face.

4. The lathe as specified by claim 1, wherein the mounting for thenon-rotatable screw-threaded member upon the subhead affords limitedaxial shifting of said non-rotatable member toward the workpiece, andsaid high-pressure hydraulic means operates to shift said non-rotatablemember within the axial limits afforded by said mounting.

5. The lathe as specified by claim 4, wherein the mounting aforesaidincludes a fluid chamber in which a portion of said non-rotatable memberis axially shiftable and exposed to pressure of hydraulic fluidintroduced into said chamber.

6. The lathe as specified by claim 1, wherein the aforesaid motor meansfor rotating the spindles comprises a pair of identical hydraulicpump-motors, one mounted upon each headstock, and each including adriving connection with a spindle; and a pipe system common to saidpump-motors for supplying thereto a pressured fluid circulation, withpressure self-equalized within said pump-motors to ensure equal sharingof the load imposed upon said pump-motors in driving the workpiece.

7. The lathe as specified by claim 6, wherein the combination includes athird hydraulic pump-motor identical to the spindle-driving pump-motors,said third pump-motor being hydraulically connected in said common pipesystem and being driveable as a pump, to supply pressurized fluid to thepump-motors first mentioned; and electric motor means for driving saidthird pump-motor as a fluid pump.

8. The lathe as specified by claim 5, wherein the aforesaid motor meansfor rotating the spindles comprises a pair of identical hydraulicpump-motors, one mounted upon each headstock, and each including adriving connection with a spindle; and a pipe system common to saidpump-motors for supplying thereto a pressured fluid with pressureself-equalized within said pump-motors to ensure equal sharing of theload imposed upon said pump-motors in driving the workpiece. t

9. The lathe as specified by claim 8,. wherein the combination includesa third hydraulic pump-motor identical to the spindle-drivingpump-motors, said third pump-motor being hydraulically connected in saidcommon pipe system and being driveable as a pump, to supply pressurizedfluid to the pump-motors first mentioned; and electric motor means fordriving said third pump-motor as a fluid pump.

10. The lathe as specified by claim 1, wherein the frictional drivemeans on the spindles comprise hard and soft laminations of a materialof the class of polyurethane, said laminations being compressiblebetween each spindle and an adjacent end face of the workpiece, with ahard lamination in contact against each workpiece end face.

11. The lathe as specified by claim 10, wherein the hard and softlaminations possess a hardness approximating Shore D-70 durometer, and ahardness approximating Shore A-60 durometer, respectively.

12. The lathe as specified by claim 1,, wherein the frictional drivemeans on the spindles comprise each a hard load ring, and a soft back-upring formed on a substantially incompressible material having theproperty of being displaceable in all directions under localizedpressure to distribute a load uniformly within itself, said rings beingcompressible between each spindle and an adjacent end face oftheworkpiece, with a hard ring in abutment upon each workpiece end face.

1. A lathe for machining a heavy elongate workpiece having oppositesubstantially planar end faces each provided with a socket coaxial withthe workpiece major axis, said lathe comprising: a stationary elongatebed having opposite end portions; a pair of headstocks slidablysupported near opposite end portions of the bed for shifting movementtoward and from one another; a drive spindle journalled for rotation ineach headstock, said spindles each having a forward spring-loaded centerto enter a socket of a workpiece end face for support of the workpieceaxially between the spindles; means precluding axial shifting of thespindles relative to their respective headstocks; frictional drive meanson the spindles adjacent to the centers thereof, for frictionallyabutting the opposite end faces of the workpiece; motor means forrotating the spindles and the abutting workpiece; a pair of sub-headsfixed upon the bed outboard of the headstocks; a pair of interconnectedscrewthreaded members located between each sub-head and its adjacentheadstock, one of said members being mounted axially in abutment againstthe rear end of a spindle for rotation independently of the spindlerotation; the other of said screw-threaded members being non-rotatablymounted upon a sub-head in alignment with said one member; a rotatableshaft coaxial with said screwthreaded members and means on said shaftfor transmitting rotation of said shaft to said one member relative tosaid other member; power means reversible for rotating the shafts ofsaid sub-heads in unison, thereby to rotate said one screw-threadedmember of each spindle relative to said other interrelated screwthreadedmember, for advancing and retracting the shiftable headstocks relativeto their respective adjacent sub-heads; and high-pressure hydraulicmeans selectively operative to augment the force applied to advance oneof the headstocks by relative rotation of the screw-threaded membersassociated with said one headstock.
 2. The lathe as specified by claim1, wherein the said high-pressure hydraulic means is operative upon oneof said screw-threaded members, for applying auxiliary force axially toa spindle and a workpiece end face abutting said spindle.
 3. The latheas specified by claim 1, wherein the said high-pressure hydraulic meansis operative upon the non-rotatable screw-threaded member to shift thelatter member in the direction of a workpiece end face.
 4. The lathe asspecified by claim 1, wherein the mounting for the non-rotatablescrew-threaded member upon the sub-head affords limited axial shiftingof said non-rotatable member toward the workpiece, and saidhigh-pressure hydraulic means operates to shift said non-rotatablemember within the axial limits afforded by said mounting.
 5. The latheas specified by claim 4, wherein the mounting aforesaid includes a fluidchamber in which a portion of said non-rotatable member is axiallyshiftable and exposed to pressure of hydraulic fluid introduced intosaid chamber.
 6. The lathe as specified by claim 1, wherein theaforesaid motor means for rotating the spindles comprises a pair ofidentical hydraulic pump-motors, one mounted upon each headstock, andeach including a driving connection with a spindle; and a pipe systemcommon to said pump-motors for supplying thereto a pressured fluidcirculation, with pressure self-equalized within said pump-motors toensure equal sharing of the load imposed upon said pump-motors indriving the workpiece.
 7. The lathe as specified by claim 6, wherein thecombination includes a third hydraulic pump-motor identical to thespindle-driving pump-motors, said third pump-motor being hydraulicallyconnected in said common pipe system and being driveable as a pump, tosupply pressurized fluid to the pump-motors first mentioned; andelectric motor means for driving said third pump-motor as a fluid pump.8. The lathe as specified by claim 5, wherein the aforesaid motor meansfor rotating the spindles comprises a pair of identical hydraulicpump-motors, one mounted upon each headstock, and each including adriving connection with a spindle; and a pipe system common to saidpump-motors for supplying thereto a pressured fluid with pressureself-equalized within said pump-motors to ensure equal sharing of theload imposed upon said pump-motors in driving the workpiece.
 9. Thelathe as specified by claim 8, wherein the combination includes a thirdhydraulic pump-motor identical to the spindle-driving pump-motors, saidthird pump-motor being hydraulically connected in said common pipesystem and being driveable as a pump, to supply pressurized fluid to thepump-motors first mentioned; and electric motor means for driving saidthird pump-motor as a fluid pump.
 10. The lathe as specified by claim 1,wherein the frictional drive means on the spindles comprise hard andsoft laminations of a material of the class of polyurethane, saidlaminations being compressible between each spindle and an adjacent endface of the workpiece, with a hard lamination in contact against eachworkpiece end face.
 11. The lathe as specified by claim 10, wherein thehard and soft laminations possess a hardness approximating Shore D-70durometer, and a hardness approximating Shore A-60 durometer,respectively.
 12. The lathe as specified by claim 1, wherein thefrictional drive means on the spindles comprise each a hard load ring,and a soft back-up ring formed on a substantially incompressiblematerial having the property of being displaceable in all directionsunder localized pressure to distribute a load uniformly within itself,said rings being compressible between each spindle and an adjacent endface of the workpiece, with a hard ring in abutment upon each workpieceend face.